This invention relates to anemometers for measuring the flow velocity of a fluid, e.g. air.
Anemometers have previously been designed for measuring wind velocity electrically. One previously proposed type is the hot wire anemometer which is of the convection cooling or heat loss type of anemometer and wherein the hot wire is usually operated so as to maintain its temperature and hence resistance at a contant value. The electric power, or voltage, needed to do this is the observed variable which is related to the wind velocity by calibration in a wind tunnel. However, in order to obtain convenient resistance values, the wire must be extremely fine and this results in it being relatively expensive, fragile and requiring frequent cleaning to maintain its calibration. If these conditions are not met then it is rather bulky, sluggish in response and requires relatively excessive power. Furthermore, it is not a simple matter to compensate it for wide variation in ambient temperature whilst the power increases considerably towards the lower end of the temperature range and in high winds.
Another previously-proposed anemometer is a thermistor anemometer. In this anemometer the thermistor resistance varies with ambient temperature and in order to obtain a measurement of the termistor power, which corresponds to a measure of the required wind velocity, it is necessary to obtain two manual settings and two meter readings, whereafter the voltage and current readings must be multiplied together. Again the power increases considerably in high winds and although this instrument can be temperature compensated over a wide range, it suffers from considerable disadvantages and cannot be regarded as effectively giving direct readings of wind velocity.
It is an object of the present invention to provide an anemometer which does not suffer from all the disadvantages of the above-mentioned anemometers and which can conveniently be constructed so as to provide a portable direct-reading battery-powered anemometer instrument.
Accordingly, the present invention provides an anemometer comprising a transistor connected in a transistor circuit, compensating means connected with said transistor circuit, and means for obtaining an output voltage representative of the difference between the forward voltage drop across the base-emitter junction of the transistor and the voltage drop across said compensating means, whereby said output voltage is substantially independent of the ambient temperature and is proportional to the temperature rise of said transistor above the ambient temperature.
More particularly, the present invention provides an anemometer comprising a transistor connected in a transistor circuit, a semiconductor diode compensating means connected in parallel with said transistor, and means for obtaining an output voltage representative of the difference between the forward voltage drop across the base-emitter junction of the transistor and the voltage drop across said semiconductor diode, whereby said output voltage is substantially independent of the ambient temperature and is proportional to the temperature rise of said transistor above the ambient temperature.